FIG. 1 illustrates a human eye in cross-section, showing cornea 13, iris 16, and lens 19. The lens 19 can suffer disease, such as a cataract, in which case surgical removal of the lens may be necessary. After removal, the lens can be replaced by an artificial lens 22, shown in FIG. 2, and which is termed an intraocular lens (IOL). The IOL restores much of the visual acuity of the eye, but has the characteristic of focusing upon the retina only those objects, such as object 29, which are within the depth of field 25 of the focusing system, which includes the IOL 22 and the cornea 13. Other objects, such as object 31 located in the far field 27, are not in focus, and thus appear blurred.
A prior art device which appears to mitigate this problem is discussed in U.S. Pat. No. 4,636,211 (Nielsen et al.) The Nielsen patent discusses a bifocal IOL as shown in FIG. 3. The Nielsen patent states that "the one piece body has a centrally located optically powered portion for near vision, designated as number 3, which is surrounded by a far vision optically powered portion 2, the two portions being concentric and coaxial."
The Nielsen patent further states that the difference in focusing power between the near vision region 3 and the far vision region 2 should be about 2.50 diopters.
The Nielsen lens, when used as IOL 22 in FIG. 2, focuses both the near-field object 29 and the far-field object 31 simultaneously upon the retina 28. Because the focus of both images differ by 2.50 diopters, the human visual system is able to reject one of the images and view the image which is preferred.
A disadvantage of the Nielsen approach occurs in the presence of bright light. Nielsen states that the diameter 40 in FIG. 3A of the near vision region 3 should be 2.12 millimeters (mm). This diameter is only slightly larger than the diameter 42 in FIG. 4 of the fully constricted pupil 44 of the average human eye when exposed to bright light. In such a situation, the fully constricted pupil prevents adequate incoming light 46 from reaching the far vision region 2 of Nielsen's lens, causing the image of the far field object 31 in FIG. 2 to become very dim on the retina 28. The fully constricted pupil inhibits clear vision of the far field object 31.
Further, even if the pupil does not fully constrict (i.e., the iris 16 does not fully cover Nielsen's far vision region 2 as assumed in the paragraph above), some visual distortion will probably occur. For example, the unblocked, or exposed, part of region 2 of the IOL will resemble annulus 47 in FIG. 5. The annulus 47 is reduced in height, which is indicated by dimension 49, which causes a reduction in cross-sectional area, which, in turn, reduces the amount of light which the annulus can collect.
This reduction in collected light causes the far field image (focused by the annulus) to be less bright than the near field image (focused by region 3). It is believed that this difference in brightness will cause the far field image to be overpowered by the brightness of the near field image, rendering the far field image useless.
The degradation of the far field image just described occurs at a time when objects in the far field are of greatest interest. That is, the bright light, which is responsible for the contraction of the pupil, is generally experienced during outdoor activities, when people are interested in viewing distant objects. However, as just shown, the pupil constriction can obstruct the far-vision region of the IOL and cause distortion or loss of distance vision.